Urea synthesis process

ABSTRACT

UREA RECYCLE PROCESS IN WHICH WATER CONTENT OF RECYCLED NH3 AND CO2 IS REDUCED AND CONVERSION IN UREA REACTOR IS INCREASED. MAJOR PORTION OF THE HEAT OF REACTION GENERATED FROM THE CONDENSATION OF THE RECYCLED NH3 AND CO2 GASES TO FORM AN AQUEOUS AMMONIUM CARBANATE SOLUTION IS RECOVERED IN HEAT EXCHANGE WITH REACTOR FEED STREAMS, TO PRODUCE STEAM IN A REACTOR COIL. HEAT GENERATED IN A UREA SYNTHESIS REACTOR IS USED IN ONE OR MORE RELATED OPERATING UNITS INCLUDING AMMONIUM CARBAMATE DECOMPOSERS, THUS PROVIDING AN EFFICIENT PROCESS HEAT BALANCE.

May 18, 1971 MAVROVIC UREA SYNTHESIS PROCESS 3 Sheets-Sheet 1 Filed llay16, 1968 mm 4 m ,n 0 w/w w n w 0 v M 1 03 W M .4 B mEDO u 18, 1971 l.MAVROVIC 3,579,636

UREA SYNTHESIS PROCESS Filed May 16, 1968 3 Sheets-Sheet 3 FIGURE 28Steam Condensate NH; C03 H 0 zoiL Ursa t- H; 0 /63 Attorney UnitedStates Patent 3,579,636 UREA SYNTHESIS PROCESS Ivo Mavrovic, 530 E. 72ndSt., New York, N.Y. 10021 Filed May 16, 1968, Ser. No. 729,764 Int. Cl.C07c 127/00 U.S. Cl. 260555 11 Claims ABSTRACT OF THE DISCLOSUREdecomposers, thus providing an efficient process heat balance.

BACKGROUND OF THE INVENTION This invention has to do with the synthesisof urea from ammonia and carbon dioxide. More particularly, improvementhas been realized in a carbamate recycle urea synthesis process byreducing the Water content of ammonia and carbon dioxide recycledtherein.

As described in US. Pats. Nos. 3,172,911 and 3,270,050, ammonia andcarbon dioxide are reacted in a reactor at elevated pressure andelevated temperature to form urea. In this synthesis an intermediatecompound, ammonium carbamate, is formed which then loses one molecule ofwater to yield urea. Only a part of the carbon dioxide charge isconverted to urea and the remainder is present in the reactor efliuentas unconverted ammonium carbamate. This unconverted ammonium carbamateand the excess ammonia (charged to improve the conversion to urea) areusually separated from the urea solution constituting the reactorefiiuent by heating the efiluent to decompose the carbamate into ammoniaand carbon dioxide, followed by expulsion of the gases from thesolution. This is usually accomplished in two sequences of decompositionand gas separation. A first decomposing vessel and a first gas separatorare usually operated at about 250-350 pounds per square inch gauge(p.s.i.g.) and 295-340 F. A second decomposing vessel and a second gasseparator are operated at 825 p.s.i.g. and 210-260" F. The resultingdegassed urea solution from the second gas separator can then beprocessed further as for the production of solid urea prills or mixedfertilizers. The ammonia and carbon dioxide gases are recovered fromeach of the gas separators and are recycled to the reactor.

Unfortunately, a relatively large amount of water is carried along withthe recovered gases, NH and CO which are usually condensed in a body ofan aqueous ammonium carbamate solution. This is particularly pronouncedwhen mole ratios of above 2.5/1 of NH /CO are employed in the reactor.The reaction system finds its own equilibrium at a relatively high watercontent in the recycle gases. This excess water is passed through thesystem to the reactor, thus reducing substantially the conversion perpass of carbamate to urea and thereby making the process uneconomical.The ammonia and carbon dioxide gases, recovered from both decomposingvessels, are usually condensed to a liquid phase and recycled back tothe reactor as an ammoniacal aqueous solution of ammonium carbamate. Theexothermic heat of condensation and reaction is usually rejected to thecooling Water and 3,579,630 Patented May 18, 1971 is removed from thesystem. This type of process is thermally very inefficient.

Only a very small amount, about 5% of the heat is recovered in such ureasynthesis processes by indirectly using a part of this heat ofcondensation to preheat the liquid NH reactor feed stream to about F.,for the purpose of bringing the reactor mixture up to 375 F. and tomaintain it at that level.

In another prior art modification, a reactor coil is used substantiallyonly for the purpose of removing the exothermic overall heat of reactionof urea formation from liquid NH and liquid or gaseous CO available in asocalled once through process, in which no carbamte is rerecycled to thereactor for recovery. The liquid NH feed to the reactor in this case isusually maintained as low in temperature as practically possible, toreduce the reactor cooling requirements.

The liquid carbamate total recycle urea process, however, at the usual3.5-4.0 to one ammonia to CO overall mol ratio in the reactor, isslightly endothermic due to the relatively cold recycle carbamatesolution which is fed to the reactor. This solution is heatedadiabatically in the reactor from about F. to about 375 F. and itabsorbs all of the exothermic heat of reaction otherwise available in aonce through urea process running on NH and CO alone, without carbamaterecycle. A relatively small amount of heat must be still added to thereactor in a total recycle process, in order to maintain the reactortemperature constant at about 375 F. This heat is normally added to thereactor by slightly preheating the liquid NH stream to the reactor.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis pro- 0 vided a process which allows for the use of higher NH COoperating molar ratios in the reactor for attaining a higher conversionper pass therein. The process comprises:

Reacting ammonia and carbon dioxide in a reactor to form a firstreaction product containing urea, Water, ammonium carbamate andunreacted ammonia,

Passing the first reaction product into a decomposing vessel and heatingit therein to decompose the ammonium carbamate to ammonia and carbondioxide and to form a second reaction product containing urea, water,ammonium hydroxide, residual ammonium carbamate, ammonia gas, carbondioxide gas and water vapor,

Passing the second reaction product into an intermediate section of aseparator wherein urea and Water are separated from ammonia, carbondioxide and water vapor, and

Recycling the ammonia and carbon dioxide to form urea, wherein A majorportion of the first reaction product is passed into the decomposingvessel, and a minor portion of said first reaction product at a lowertemperature than the second reaction product is passed into an uppersection of said separator countercurrent to the gaseous phase of saidsecond reaction product passing upwardly therein, whereby a substantialportion of the water vapor of said second reaction product is condensedand is separated from the ammonia and the carbon dioxide of the gaseousphase of said second reaction product.

Furthermore, in accordance with the present invention, a major portionof the heat of recycle NH, and CO condensation is utilized within theprocess and is exchanged internally to produce valuable steam at ausable temperature level. Instead of rejecting this heat of condensationat a low temperature level via the cooling water (at 110-120 F.) thisinvention utilizes the major portion of this heat of condensation andrecycles it back to the reactor via the reactor feed streams to producein intermediate section of second separator 43, which is equipped with atray section 44.

A minor portion of stream 35 is passed through line 45 and flow controlvalve 46. As the stream is flashed through valve 46, the temperaturethereof is lowered as from 320 F. to about 210 F. The cooler streamflows from valve 46 through line 47 to an upper section 48 of separator43. In separator 43, the cooled stream flows countercurrent to theupwardly flowing gaseous phase of the third reaction product thereinand, as a result, the major portion of water vapor in the gaseous phaseis condensed. An aqueous solution of urea containing a small amount ofbiuret (formed as a by-product of urea), residual ammonium carbamate andammonia, is removed from separator 43 through line 49 containing levelcontrol valve 50. Urea can be removed from this mixture by conventionalmeans (not shown). The mixture comprises, in weight present:

Urea 7 50 H O 23.1 NH 0.6 H NCO(ONH 1.0 Biuret 0.3

The overhead product removed from separator 43 through line 51 containsNH CO and a relatively minor amount of water, 30 mole percent, incontrast to 65 mole percent of water in the gaseous phase of the thirdreaction product passed into separator 43. The overhead product, at 220F., is passed into a lower section of absorber 52 and flows upwardlytherein in counter-current relation to a circulating solution of aqueousammoniacal carbamate solution, which condenses and absorbs the gaseousphase. The latter solution, at 90 F., is passed into absorber 52 throughline 53 containing spray nozzle 54, and is removed through line 55,containing pump 56 to cooler 57 The ammonium carbamate solution incooler 57 is cooled by water at 80 F, passed through the jacket thereofthrough inlet 58 and outlet 59. Inerts, principally N and H are removedfrom absorber 52 through overhead line 60 containing valve 61.

A portion of the ammonium carbamate solution in line 55 is pumped bypump 62 through line 63 to line 64 wherein it is combined with theoverhead gas from separator 29 in line 34. The combined stream in line64 is passed into a lower section of absorber 65 and the gaseous phaseflows upwardly therein to contact a countercurrent stream of refluxliquid ammonia which is passed into absorber 65 through line 66.Absorber 65 is shown with reflux section 67 positioned in an uppersection thereof. Aqueous ammonium carbamate enriched with NH andadditional carbamate, and containing only a minor amount of water isremoved from absorber 65 through line 18 and thence pump 19 and line 20to reactor 10. The solution in line 18 comprises, in weight percent:

H2NCO(ONH4) 56.7 NH3 23.3 H2O 20.0

Substantially pure ammonia gas separated from CO is taken overhead fromabsorber 165 through line 68 containing valve 69 to condenser 70. Coldwater at 80 F. is circulated through the shell of condenser 70 via lines71 and 72 to control the temperature thereof. Liquid ammonia is passedfrom condenser 70 through line 73 to ammonia storage 74 from which itcan be taken through units 14-17 to reactor 10. Liquid ammonia is addedto storage 74 through line 75 containing valve 76. Inerts, such as N CHH and O in storage tank 74 by virtue of their presence in ammoniastreams 73* and 75, can be removed from the system through line 77controlled by valve 78.

As shown in connection with absorber 65, a small amount of steamcondensate at 100 F. is added from line 6 79 above tray section 67therein. Such condensate is an aid to improved ammonia gas purificationfrom CO in tray section 67 of absorber 65.

As shown in this example, illustrated by FIG. 1, a conversion of 68-70mole percent of CO can be maintained by reducing the water content ofstreams 34 and 51 such that the water content of stream 20 isapproximately 34-35 mole percent.

Comparative Example A When streams 31 and 45 are eliminated, and theentire streams 21 and 35 are introduced into decomposing vessels 2 4 and38, respectively, the water contents of streams 34 and 51 areundesirably high, with the result that stream 20 has a water content ofabout 40 mole percent. Further, the conversion in reactor 10 is onlyabout 60-62 mole percent of CO Otherwise all conditions aresubstantially the same as in Example 1.

Example 2 As illustrated by FIG. 2, carbon dioxide (4416 parts) ispassed from line to compressor 101 operated by motor 102, to line 103and into the bottom of high pressure reactor 104. Liquid NH (7230 parts)in line 105 and ammoniacal carbamate solution in line 106 are alsopassed into the bottom of reactor 104. The reactor is provided withsteam producing coil 107, which is immersed in the resulting reactionmixture therein, for the purpose of removing the exothermic heat ofreaction and thus of maintaining the reactor operating temperatureconstant. Steam condensate in line 108 is delivered by pump 109 to coil107, and the resulting steam is passed from coil 107 to line 110 andthence to steam drum 111. Steam so produced can be flashed from drum 111through line 112 for use in one or more other operating units in theprocess as described hereinafter.

Reaction temperatures and pressures in reactor 104 are about 370 F. andabout 3200 p.s.i.g., respectively.

The reactor eflluent stream 113 has the following composition, in weightpercent, indicating an overall CO conversion per pass of 70 percent:

Urea 34.4 CO 10.8 NH 37.8 H O 17.0

A major portion (about 80 percent by volume) of stream 113 is passedthrough line 114 containing pressure control valve 115, which serves tomaintain reactor pressure constant in reactor 104. The pressure of thestream in line 114 is reduced from 3200 p.s.i.g. to 330 p.s.i.g. inpassing through valve 115; this stream is then passed into firstdecomposing vessel 116 which is operated at about 320 p.s.i.g. and 320F. Steam, 150 p.s.i.g., is passed from line 112 through reactortemperature control valve 117 to line 118 and to the shell 119 ofdecomposer 116 and thus heats the latter.

The flow of additional or make-up steam is supplied to line 118 fromline 120 controlled by temperature control valve 121 which, in turn, iscontrolled by the temperature of the solution in the lower portion ofseparator 122. A suflicient quantity of steam is supplied from line 120to maintain the said temperature in separator 122 at about 320 F.

The stream in line 11 4, upstream of valve 115, contains In decomposer116, the ammonium carbamate is heated and is decomposed into NH and COExcess NH and the gases formed from the carbamate decomposition areexpelled from the urea solution in decomposer 116. Thus, a gaseous phasecontaining about 14 percent by The urea solution formed as a liquidphase in separator 152 comprises, in weight percent:

Urea 75.8

H NCO(ONH 0.4 NT-I 0.2 H O 23.4 Biuret 0.2

This urea solution is removed from separator 152 by means of line 160,booster pump 161 and line 162 containing level control valve 163. Thisvalve serves to maintain a substantially constant liquid level inseparator 152. The urea solution in line 162 can be routed to a ureafinishing section (not shown) for further processing to solid ureaprills, or to storage.

Returning to the gaseous phase separated in separator 152, containing 4weight percent CO 14 weight percent NH and 82 weight percent watervapor, this phase is removed through overhead line 164. In contrast tothe gaseous phase having a water vapor content of about 86 percent byvolume at the intermediate section of separator 152, the gaseous phasein line 164 has a water vapor content of about 82 percent by volume.Gases in line 164 are passed to venturi mixing nozzle 165, then to line166 and into an upper section of shell and tube, falling film heatexchanger 154. Passed into venturi mixing nozzle 165 is a liquidsolution taken from the bottom of exchanger 154 and recirculated throughvalved line 167 by means of pump 168. Nozzle 165 serves to mixintimately the solution in line 167 with gases from line 164 for betterreaction and to create a slightly lower pressure in separator 152 withrespect to the internal pressure (about 1 p.s.i.g.) in condenser 154. Asa result, more effective degassing of urea product in separator 152 isrealized and, therefore, a superior NH and CO recovery from the ureaproduct solution in separator 152 results.

In exchanger 154, gases from line 164 are absorbed substantiallycompletely and are condensed in the solution from line 167. Theexothermic heat of reaction between the gaseous and liquid phases andthe exothermic heat of water condensation, are absorbed by a stream ofcooling water flowing through the shell side of condenser section 154.Accordingly, stream 164 is substantially completely condensed and theresulting solution is cooled to about 100 F. before it is removed fromexchanger 154 through line 167 and a portion thereof is recirculated asmentioned above. Recirculation is used to facilitate cooling andcondensation of gases from line 164, and to facilitate indirect transferof the exothermic heat of gas condensation to stream 171-172 of coolingwater flowing through the shell of exchanger 154.

A relatively small amount (1 percent) by volume of inerts comprising Nand CH which can accumulate in the gaseous phase formed above the inletof line 166 of exchanger 154, enters tray section 169 (or equally, abubble cap section). The inerts flow upwardly and countercurrently towater introduced from line 170; they are washed free of traces of NH andCO and are vented to the atmosphere through line 173.

The liquid level in the bottom section of exchanger 154 is maintainedsubstantially constant by withdrawing a regulated portion of thesolution in line 167 and passing it through line 174 containing levelcontrol valve 175. The solution in line 174 contains, in weight percent:

NH 12 co 4 H2O s4 Analogous to treatment given to the gases in line 164as described above, the gases from separator 138 and removed throughline 143 are mixed in venturi mixing nozzle 176 with recirculated liquidin valved line 177 pumped by pump 178. The resulting mixture is passedthrough line 179 to the top of vertical falling film, shell and tubeheat 10 exchanger section 180 of vessel 181. The gases in line 143comprise, in approximate weight percent:

NH 62 C0 15 H2O 23 The gases from line 143 are substantially completelyabsorbed and condensed upon contact with the solution passed into vessel181 from line 174. Exothermic heat of reaction between the gases fromline 143 and liquid from line 174, and exothermic heat of watercondensation, are absorbed by the stream of line 177 and are transferredto a stream of cooling water, at F., flowing from line 182 through theshell side of vessel 181 to line 183. Thus, stream 143 is substantiallycompletely condensed and the resulting solution thereof with stream 174is cooled to about F. before leaving vessel 181 through line 177.

A portion, about 10 percent by volume, of the solution recirculatedthrough line 177 is passed to line 184 and valved line 185 and is fed toa central portion of tray section 186 of vessel 181. The portion sodirected serves to countercurrently wash inerts which may accumulate inthe gaseous phase formed at the top of vertical falling film exchangertubes of vessel 181. The inerts are also washed countercurrently by thesolution from line 174 which is fed to the top tray of tray section 186.The inerts are purged from vessel 181 through line 187 containingpressure control valve 188, and are passed to venturi mixing nozzle 165.Such inerts are purged from the system through line 173.

The liquid level of the bottom section of vessel 181 is maintainedsubstantially constant by withdrawing a regulated amount of solutionfrom circulating stream 177 and directing it through line 189, thenthrough booster pump wherein the pressure thereof is increased fromabout 25 p.s.i.g. to about 330 p.s.i.g. The solution in line 189contains, in approximate parts per weight:

NH3 42 CO2 11 H2O 47 The solution is discharged from pump 190 throughline 191 and is split into five streams into individually valved lines192, 193, 194, 195 and 196.

With regard now to gas stream 128 containing, in approximate weightpercent, 21 CO 74 NH; and 5 water vapor, this stream is fed to the shellside 197 of vertical shell and tube heat exchanger 198. Gas stream 128is mixed with liquid stream 192 to which has been added liquid stream199 (source of which is described hereinafter), before it is passed intoshell side 197. The gas stream, 128, is partially absorbed in the bodyof liquid present in shell side 197, which is maintained at about 265 F.The flow of stream 192 is regulated to provide for the highesttemperature possible in shell side 197, since an excessive amount ofcold stream 192 would cause cooling of the solution in shell 197 as muchas would an insufficient amount of stream 192.

Ammonium carbamate and aqueous ammonia solutions are formed in the shellside 197 and heat of reaction thereof is transferred to the tube side ofexchanger 198, to which steam condensate is introduced from line 200.The water is evaporated to steam in the tube side and steam is removedthrough line 201. Part of the steam so produced is passed from line 201to line 202, temperature control valve 203 and line 134, to the shellside 135 of decomposer 133. The flow of steam in line 202 is regulatedby temperature control valve 203 which, in turn, is controlled by thetemperature of stream 144 leaving separator 152. Another portion of thesteam in line 201 is directed through line 204, temperature controlvalve 205 and line 148 to be passed into the shell side 149 of seconddecomposer 147.

The steam condensates in the shell side 135 of decomposer 133, and inthe shell side 149 of decomposer 147,

exchanger 262 via line 223 at about 120 F. and it is then passed to thetube side of exchanger 222. It is heated therein to about 245 F. and isthen passed through line 224 as mentioned earlier. The condensateportion in line 229 is passed to line 264, wherein it is combined withsteam and inerts (N and from line 227, and the combined stream in line264 is passed to tank 207.

An additional stream of condensate may be introduced into stream 263 vialine 265, or into separator 111 via line 266, or into both. Suchadditional condensate is withdrawn from the main 'body of condensateformed in the subsequent urea evaporation or crystal-melting prillingsection (not shown) in a treatment of the urea solution from line 162.The additional condensate usually issues from heat exchangers in suchevaporation or crystal-melting section at about 320 F., is flashed to anominally 30 p.s.i.g. condensate collecting tank and is returned to themain boiler plant (not shown) at about 250 F. Recircu lation of theadditional condensate, supplied at about 320 F. through one or both oflines 265 and 266, through heat exchangers 262 and 222, respectively,provides superior heat recovery within the urea synthesis systemillustrated by FIG. 2. The greater the amount of condensate in line 263,the greater amount of heat added to the liquid NH feed stream 105 and ofsteam produced in reactor coil 107, the greater the amount of heatexchanged in exchanger 222 and the smaller the amount of heat rejectedto the cooling Water stream flowing through the shell side 235 ofexchanger 236.

While CO is shown as compressed to the reactor pressure and introducedinto reactor 104 at its compressor discharge temperature (usuallybetween about 250 F. and about 300 F.), alternative procedures may beused in charging it to reactor 104. It may be charged directly theretoas a liquid. It may also be compressed to reactor pressure and thencooled to about 140 F. in a C0 compressor discharge cooler (not shown),in order to remove any oil present in the gaseous CO In this alternativeprocedure, this relatively cooler CO is preheated to about 250 F. in anadditional heat exchanger (not shown) interposed between heat exchangers222 and 236 in FIG. 2.

By way of comparison of the process illustrated by FIG. 2 with a priorart total urea recycle process, the following advantages in utilitiesobtain:

In comparison with the process illustrated in Example 2 and FIG. 2, thegaseous phase of stream 127 could be reduced in water content to asatisfactory level, one comparable to that of stream 128, by directingthe entire stream 113 to the top of tray section 126 of first separator122. Thus, stream 114 would be eliminated, and the bottom solution instream 129 would have to be charged to first decomposer 116 in order tobe heated and degassed therein. In such case, decomposer 116 would serveas a re-boiler instead of a once through high liquid velocitydecomposer. Obviously, the liquid retention time of the urea solution ina re-boiler would be substantially longer than in decomposer 116 asshown in FIG. 2 due to a lower differential head (pressure) availableand thus a lower tube velocity. With such a relatively longer retentiontime, there is greater opportunity for urea to form biuret. In suchcase, the biuret content of the urea solution withdrawn from the systemthrough line 162 is of the order of about 0.5 percent by weight, incontrast to about 0.2 percent by weight for the solution in stream 162of FIG. 2.

It is to be understood that the above description, together with thespecific examples and embodiments de- Comparative Example 14 scribed, isintended to illustrate the invention, and that the invention is not tobe limited thereto, nor in any way except by the scope of the appendedclaims.

I claim:

1. In a urea synthesis process for forming urea by reaction of ammoniaand carbon dioxide, wherein ammonia and carbon dioxide are reacted in areactor to form an aqueous urea solution (A) containing ammoniumcarbamate and ammonia,

said aqueous urea solution (A) is split into a major portion thereof anda minor portion thereof,

said minor portion of said aqueous urea solution (A) is cooled,

said major portion of said aqueous urea solution (A) is passed to afirst ammonium carbamate decomposer wherein ammonium carbamate isdecomposed at a temperature of from about 295 F. to about 320 F. toammonia and carbon dioxide and the resulting gaseous phase containingammonia and carbon dioxide gases are expelled from said aqueous ureasolution (A) together with water vapor and ammonia in said solution (A),and forming a first decomposition product (D) comprising a gaseous phase(B) containing ammonia gas, carbon dioxide gas and water vapor and aliquid phase (C) comprising an aqueous urea solution containing residualammonium carbamate and urea, the improvement which comprises thesequential steps of passing said first decomposition product (D)containing said gaseous phase (B) and said liquid phase (C) into a lowersection of a separator, wherein said gaseous phase (B) is separated fromsaid liquid phase (C) and rises in said separator, said separator beingmaintained at a pressure of from about 250 to about 350 p.s.i.g.,passing said minor portion of said aqueous urea solution (A) at a lowertemperature than said gaseous phase (B) into an upper section of saidseparator countercurrently to said separated rising gaseous phase (B) toheat said minor portion of solution (A) to a temperature above thedecomposition temperature of ammonium carbamate in the range of fromabout 295 F. to about 320 F. to decompose ammonium carbamate in saidminor portion of solution (A) to ammonia and carbon dioxide, and to coolsaid gaseous phase (B), removing said gaseous phase (B) and recoveringthe resulting aqueous urea solution. 2. The process of claim 1, whereinsaid ammonium carbamate decomposition product is condensed in indirectheat exchange relation with a steam condensate which is at a pressure offrom about 15 to about 25 p.s.i.g. to produce steam, and

passing said resulting steam at a pressure of from about 15 to 20p.s.i.g. in indirect heat exchange relation with an aqueous ureasolution (B) containing ammonium carbamate at a temperature of about 250F. and a pressure of from about 0 to about 25 p.s.i.g., to decomposesaid ammonium carbamate to ammonia and carbon dioxide. 3. The process ofclaim 1 including the sequential steps of splitting an aqueous ureasolution (E) containing residual ammonium carbamate and ammonia andobtained from a preceding ammonium carbamate decomposer into a majorportion (F) thereof and a minor portion (G) thereof,

cooling said minor portion of said aqueous urea solution (G),

passing said major portion of said aqueous urea solution (F) to asubsequent ammonium carbamate decomposer wherein residual ammoniumcarbamate is decomposed to ammonia and carbon dioxide and the resultingamomnia and carbon dioxide gases are expelled from said portion of saidaqueous urea solution (F) together with ammonia therein, as a subsequentdecomposition product comprising a gaseous Patent No. 5791636 D t d May18, 197].

I IVO MAVROVIC It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 3, line 54- in the tabulation heading "Preferrep" sh0u1d berewritten as "Preferred", as indicated in specification at page 8, line7.

Column 5, line 19 "7 .50" should be rewritten as ."75.0"

as indicated in specification at page 12, line 13.

Column ll, line 10 delete entire line and reinsert between lines 5 and 6as indicated in specification at page 28, lines 7-8.

Column 13, line 48 "300" should be corrected to read "130",

as indicated in specification at page 35, line 6.

Patent No.

Inventor(s) UNITED STATES PATENT OFFICE 3, 579,636 6 May 18, 1971 DateIVO MAVROVIC PAGE 2 It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 14,

Column 15,

line 72, Claim 3 after "resulting", "amomnia" should be rewritten as"ammonia", as indicated in amendment filed April 29, 1970 at page 6,line 12 of claim 19.

line 4, Claim 3 after "ammonia" the period as indicated in should bechanged to a comma amendment filed April 29, 1970 at page 7, line 19 ofclaim 19.

Signed and sealed this 7th day of December 1971 (SEAL) Atte st:

EDWARD M.FLETCHER,JR. Attesting Officer ROBERT GOTTSCHALK ActingCommissioner of Patents

